While surface features are readily seen when a surgeon views human or other mammalian tissue, whether through a microscope or with the unaided eye, subsurface features may be hidden. Locating and understanding subsurface features may, however, be important for treatment and operative decisions. While light of some wavelengths can penetrate much mammalian tissue, including human brain and breast tissue, to a greater depth than that at which features are readily discernible, the tissue is turbid, scattering the light such that subsurface features may not be easily seen.
There are other fields, from meat inspection and navigation of vehicles through fog, to noninvasive testing of plastic parts including composite aircraft parts, where improved visualization of subsurface features in turbid material may also be of use.
A mammalian, including human, brain 100 typically has a network of blood vessels 102 on the brain surface 104. Generally, brains have a network of fissures 106, known as sulci, and are covered by membranes 108, known as the meninges and including the dura-mater. Beneath the brain surface 104 is typically a layer of grey matter 110, often containing cell bodies of neurons with supporting glia cells, over white matter 112, containing interconnect nerve axons. The brain includes many nerve tracts and nuclei 114 that have particular functions, including the substantia nigra, corticopontine tracts, and many others.
All too many brains may have a tumor 116, cystercerci, or foreign objects for which surgical removal may be desirable. Tumors, especially malignant tumors, also often have invasive processes 118 that can penetrate white and/or grey matter for a considerable distance from the main body of the tumor 116. Brains may also have fluid-filled cavities 120, such as ventricles. Some of these structures have color, turbidity, or other optical characteristics, that differ from other surrounding tissues.
The gray-white matter boundary, tumors 116, processes 118, cavities 120, cystercerci, nerve tracts, and nuclei are inhomogeneities within the brain—regions that have one or more absorbance or scattering parameters that differ from other regions within the brain.
Even after opening of the meninges 108, only brain surface 104, sulci 106, and surface blood vessels 102 may be visible to the unaided eye.
When a tumor 116 is removed, high quality surgery involves removing as much of tumor 116 body and processes 118 as possible, while avoiding removal of excessive normal white or gray matter, and avoiding excessive, unwanted, or unintended collateral damage to underlying structures such as nerve tracts and nuclei 114. A system providing enhanced visibility of subsurface features, including tumor 116 body, tumor processes 118, tracts and nuclei 114, without exposing surgeons and other operative personnel to x-ray radiation, may improve surgical quality.
Andrea Bassi, et al., Spatial shift of spatially modulated light projected on turbid media, Opt Soc Am A Opt Image Sci Vis. 2008 November; 25(11): 2833-2839, have proposed that processing images of tissues when illuminated by structured, or spatially modulated, light can be of use in determining the absorption (pa) and the reduced scattering (μ's) coefficients for diagnostic applications. In this article, Andrea fails to describe incorporation of imaging under structured light into a surgical microscope, and fails to describe using such processing in real time during surgery.
“Structured light” generally refers to light patterns exhibiting some type of periodicity over the field of view.